Dirction of fastest growth

I have f(x,y,z)=(x^2)y-x(e^z) and point Po=(2,-1,pi)
I need to find
a) gradient at point Po ( done)
b) Rate of change of f at point Po in the direction of vector u=i-2j+k (it's also done)
c) Unit vector in the direction of fastest growth of f at Po.

I can't find formulas for a last on. Does it come from a) and b)?
I know that angle should be zero but I am not sure what angle it is.

The gradient is the "derivative" of a function of 2 or more variables. The derivative of f(x,y) in the direction of angle [itex]\theta[/itex] if given by
[tex]D_\theta f(x,y)= (cos \theta i+ sin \theta j)\dot(\frac{\partial f}{\partial x}i+ \frac{\partial f}{\partial y})[/tex]
That will have a maximum (with respect to [itex]\theta[/itex]) where it's derivative with respect to [itex]\theta[/itex] equals 0:
[tex]-cos\theta \frac{\partial f}{\partial x}+ sin\theta\frac{\partial f}{\partial y}= 0[/tex]
That means that
[tex]tan\theta= \frac{sin\theta}{cos\theta}= \frac{\frac{\partial f}{\partial x}}{\frac{\partial f}{\partial y}}[/tex]

Think about what that means in terms of the components of the gradient of f.